The simple way to look at this is that medical necessity is the overarching criterion for any service that is billed to insurance. So if there is no medical necessity for a service performed, then that service should not be billed to insurance. The cost incurred to the patient is not the concern, even if the doctor feels that it is. If these U/S's are not medically necessary, then they should not be billed to insurance, and if the doctor wants to eat the cost of doing them, that is their choice.
Under-coding is something else entirely... for example: Performing a medically necessary procedure and just billing an E/M code, or performing a higher level E/M code such as a 99214, and reducing it down to a 99212, for instance.